This study demonstrates that both mild heat stress that do not affect foliage photosynthetic characteristics; and severe heat stress that lead to irreversible photosynthetic reductions resulted in changes in emissions of mono- and sesquiterpenes in tomato (S. lycopersicum) leaves immediately after heat stress and during recovery. The analysis overall underscores the complexity of linkages between gene-level and emission-level responses, especially in the case of severe heat stress that results in non-recoverable perturbation of leaf physiological activity. Such complexity can be partly considered in process-based emission models simulating terpenoid emissions in dependence on gene expression by introducing a “delay factor”. However, further studies are needed to gain insight into what controls heat-dependent regulation of transcription activity of terpenoid synthases, and how the delay between gene expression and phenotypic response scales with the severity of heat stress.

Plants frequently experience heat ramps of various severities, but how and to what degree plant metabolic activity recovers from mild and severe heat stress is poorly understood. In this study, we exposed the constitutive terpene emitter, Solanum. lycopersicum leaves to mild (37 and 41 °C), moderate (46 °C) and severe (49 °C) heat ramps of 5 min and monitored foliage photosynthetic activity, lipoxygenase pathway volatile (LOX), and mono- and sesquiterpene emissions and expression of two terpene synthase genes, β-phellandrene synthase and (E)-β-caryophyllene/α-humulene synthase, through a 24 h recovery period upon return to pre-stress conditions. Leaf monoterpene emissions were dominated by β-phellandrene and sesquiterpene emissions by (E)-β-caryophyllene, and thus, these two terpene synthase genes were representative for the two volatile terpene classes. Photosynthetic characteristics partly recovered under moderate heat stress, and very limited recovery was observed under severe stress. All stress treatments resulted in elicitation of LOX emissions that declined during recovery. Enhanced mono- and sesquiterpene emissions were observed immediately after the heat treatment, but the emissions decreased even to below the control treatment during recovery between 2 and 10 h, and raised again by 24 h. The expression of β-phellandrene and (E)-β-caryophyllene synthase genes decreased between 2 and 10 h after heat stress, and recovered to pre-stress level in mild heat stress treatment by 24 h. Overall, this study demonstrates a highly sensitive heat response of terpenoid synthesis that is mainly controlled by gene level responses under mild stress, while severe stress leads to non-recoverable declines in foliage physiological and gene expression activities.